Laboratory Evaluation of In-Situ Stress Contrast in Deeply Buried Sediments

Abstract

ABSTRACT Delineation of the effects of in-situ stress contrasts on fracture containment requires the field measurement of the minimum horizontal in-situ stress of the pay zone and adjacent layers. Small hydraulic fracturing tests are presently the most practical method with which to determine the state of stress acting at great depth within the earth. Several attempts have been made to correlate the state of stress of a rock layer and its current mechanical properties. Recently techniques for the independent determination of P- and S-waves during wellbore logging have been implemented by the logging service industry. Such advances help establish a more reliable value for the in-situ rock elastic properties, namely Young's modulus and Poisson's ratio. Higher Poisson's ratio value has been proposed as an indicator of higher minimum horizontal in-situ stresses. This is compatible with trends for reservoirs in the Louisiana/Gulf of Mexico area where clean sandstone pay zones are prevalent. Burial history and rock constituents may alter such conclusions. Although correlation with Poisson ratios of the various formations may provide rough estimates of horizontal stress gradients, they are inadequate to ascertain which of two adjacent formations has the higher horizontal stress, especially in tight reservoirs where the clay content in the sandstone layers is high and the differences between sandstone and shale layers diminish. Inelastic deformation, as well as changes in material properties and strain during the burial history have a significant influence on today's state of stress at depth. Furthermore, correlation of direct in-situ stress measurements with general laboratory response of granite, sedimentary rocks and salt formation to applied loads imply that soft, high ductility material (higher principal strain ratio during inelastic flow) as well as materials not capable of sustaining large deviatoric stress (low ratio of deviatoric stress to mean stress in uniaxial strain deformation) possess higher minimum horizontal stresses (cf. Abou-Sayed, et al., 1981). In the absence of direct, field-measured in-situ stress data, special laboratory tests are presented here with analysis to provide a suggestive basis for evalution of in-situ stress contrast within different layers for fracture containment analysis. The laboratory tests presented in this paper are aimed at providing the mechanical properties of the rock layers under two deformation histories: 1) triaxial compression tests to determine the elastic moduli of rocks, and 2) one-dimensional (uniaxial) strain tests to delineate the long-term correlation between horizontal and vertical stresses within the various layers. Tests to measure the coefficient of thermal expansion of rocks under simulated in-situ conditions may also be performed. The laboratory-deduced stress contrasts in various rock types are compared to field-measured data. Several case studies are presented and close agreements are observed.